1. Field of the Invention
The present invention relates to a ceramic ball, a method for manufacturing the same, a ceramic ball bearing, and a check valve.
2. Description of Related Art
A ceramic ball exhibits excellent wear resistance as compared with a metallic ball, and is thus used as a ball bearing. For example, Japanese Patent Application Laid-Open (kokai) No. 2000-74069 discloses a bearing including inner and outer rings made of high carbon chromium steel or stainless steel, and zirconia balls serving as rolling elements. According to the disclosed configuration, since the coefficient of linear expansion of the inner/outer ring is substantially equal to that of the zirconia ball, variation in pre-load caused by variation in temperature is reduced. Thus, even when the bearing is applied to precision equipment, such as a hard disk drive for computer use, rotational precision can be maintained high. As disclosed in, for example, Japanese Patent Application Laid-Open (kokai) No. 11-153142, such a zirconia ball bearing is manufactured by the steps of: forming a green body by means of a uniaxial press or a hydrostatic press, the slip casting process, or the injection molding process; subjecting the green body to preliminary firing at atmospheric pressure so as to obtain a relative density of not less than approximately 95%; and subjecting the fired body to hot isostatic pressing (HIP).
In the field of a check valve disposed in a fluid path so as to limit fluid flow within the fluid path to a single direction, a ceramic ball of silicon nitride or like material is used as a valve in equipment for filling bottles and cans with drink and apparatus operating at high speed and high frequency, such as a weft insertion plunger pump for use in a water jet loom.
The unidirectional pressing process is most generally used to form a spherical green body in manufacture of a conventional zirconia bearing ball. However, the process involves a problem in that friction and bridging between a die and powder causes a formed green body to suffer nonuniform density (i.e., nonuniform distribution of pores) and a number of large pores remaining locally. The problem is eased to some extent by means of subsequent cold isostatic pressing. However, complete removal of pores is difficult. A green body formed by means of the slip casting process or the injection molding process is essentially of low density; i.e., pores are much more likely to remain.
According to many patent publications regarding zirconia ceramic, through employment of HIP, large pores remaining in a presintered body are crushed to thereby obtain ceramic of very small pore size. For example, Japanese Patent Application Laid-Open (kokai) No. 62-235255 discloses a zirconia sintered body having a pore size of not greater than 0.1 xcexcm. The techniques disclosed in the publications are not intended for formation of a spherical green body. Thus, the publications do not refer to the results of firing of a spherical green body. Conceivably, the disclosed techniques employ a forming shape, such as rectangular parallelepiped or circular cylinder, that allows uniform pressing in a relatively easy manner and is unlikely to generate a defect. In manufacture of a zirconia ceramic ball for use in, for example, a bearing, the conventional techniques unavoidably encounter the above-mentioned problem of residual pores derived from a forming process and thus fail to yield a sintered body of small pore size even when HIP is employed.
When the above-mentioned conventional techniques are applied to manufacture of ball bearings having a small diameter of not greater than 5 mm for use in a hard disk drive or like apparatus, the following problem will arise. Since the density of a green body formed by means of the conventional techniques becomes lower and more nonuniform, presintering encounters difficulty in attaining a relative density of not lower than 95%. Thus, attainment of a relative density of not lower than 98% is impossible. Hence, a sintered body of small pore size cannot be obtained.
Japanese Patent Application Laid-Open (kokal) No. 2000-2350 discloses a silicon nitride ceramic ball for use as the valve element of a check valve. In order to improve wear resistance, the silicon nitride ceramic ball can be replaced with a zirconia ceramic ball. However, the replacement also unavoidably involves the above-mentioned problem of residual pores derived from a forming process.
The present invention provides a zirconia containing ceramic ball formed of a zirconia containing ceramic material which contains in an amount of not less than 10% by volume of the zirconia ceramic phase predominantly comprising zirconium oxide. The size of the largest pore (hereinafter called the maximum pore size) among pores present in a region extending radially from the surface of the ball to a depth of 50 xcexcm (hereinafter called the surface layer region) as observed on a polished cross section of the ball taken substantially across the center of the ball is not greater than 3 xcexcm.
The above-mentioned zirconia containing ceramic ball having a maximum pore size of not greater than 3 xcexcm can be manufactured by the following method of the present invention. The method comprises:
a rolling granulation process for obtaining a spherical green body having a relative density of not less than 61%, the rolling granulation process comprising the steps of: preparing a forming material powder which contains in an amount of not less than 10% by volume a zirconia ceramic phase predominantly comprising zirconium oxide; placing the forming material powder in a granulation container; and rolling an aggregate of the forming material powder within the granulation container such that the aggregate grows into a spherical body; and
a firing process for firing the spherical green body to obtain a zirconia containing ceramic ball formed of a zirconia containing ceramic material which contains in an amount of not less than 10% by volume a zirconia ceramic phase predominantly comprising zirconium oxide, the maximum pore size among pores present in the surface layer region as observed on a polished cross section of the ball taken substantially across the center of the ball being not greater than 3 xcexcm.
As mentioned previously, in manufacture of a zirconia containing ceramic ball, a green body formed conventionally by means of the uniaxial pressing process or the cold isostatic pressing process contains a number of large pores remaining as pores. The present inventors conducted extensive studies and found that increasing the density of a powder green body to a possible extent and thereby lessening the degree of nonuniform density is important for solving the above-mentioned problem and that imparting a relative density of not less than 61% to a green body is effective for obtaining the above-mentioned green body of low nonuniformity of density. Such a highly dense green body can hardly be manufactured by the pressing process or the cold isostatic pressing (CIP) process but can be easily manufactured by the above-mentioned rolling granulation process. The present inventors found that firing the thus-obtained spherical green body yields the above-mentioned zirconia containing ceramic ball that attains a conventionally unattainable level of pore size; specifically, a maximum pore size of not greater than 3 xcexcm and thus have achieved the present invention. The rolling granulation process exhibits high efficiency in manufacturing a spherical green body and can avoid the problem of providing increased allowance for polishing, since, in contrast with the case of pressing, an unnecessary flange portion is not formed on a green body.
Thus it is an object of the present invention is to provide a zirconia containing ceramic ball of smaller pore size and excellent durability as compared with that manufactured by conventional methods.
It is a further object of the present invention to provide a method of manufacturing a zirconia containing ceramic ball of smaller pore size and excellent durability.
It is a still further object of the present invention to provide a ceramic ball bearing using zirconia containing ceramic balls of smaller pore size and excellent durability.
It is a still further object of the present invention to provide a check valve using a zirconia containing ceramic ball of smaller pore size and excellent durability.
Further objects and advantages of the present invention will become apparent by reference to the following description of the preferred embodiment and appended claims wherein like reference numbers refer to the same component, element or feature.